Hyponatremia

Introduction

Hyponatraemia is a serum sodium level of less than 135 millimoles per litre. It is condition that occurs when the level of sodium in the blood is too low.

Plasma Na+<135mmol/L = Hyponatraemia

Aetiology

There are different types of hyponatraemia with varied aetiologies
Pseudo-hyponatraemia:

  • With normal plasma osmolality as seen in hyperlipidaemia or hyper proteinaemia
  • With increased plasma osmolality as seen in hyperglycaemia, infusion of mannitol

Hypo-osmolar hyponatraemia:

  • Due to a primary water gain and secondarybsodium loss, or a primary sodium loss and secondary water gain
  • Integumentary loss: sweating, burns
  • Loss from the GIT: vomiting, tube drainage, fistula
  • Renal loss: diuretics, hypoaldosteronism, salt wasting neuropathy, obstructive diuresis
  • Primary polydypsia
  • Cardiac failure
  • Hepatic cirrhosis
  • Nephritic syndrome
  • Decreased solute intake: SIADH (Syndrome of inappropriate antidiuretic hormone secretion)
  • Glucocorticoid deficiency
  • Hypothyroidism
  • Chronic renal insufficiency

Clinical features

  • Cerebral oedema.
  • May be asymptomatic
  • Otherwise nausea, malaise, headache, lethargy, confusion, and altered consciousness
  • Coma when plasma sodium is less than 120 millimoles per litre

Differential diagnoses

  • Congestive cardiac failure
  • Hepatic cirrhosis
  • Nephritic syndrome

Investigations

Directed at establishing the cause and severity of hyponatraemia

Treatment objectives

  • To correct plasma sodium concentration by
    restricting water intake and promoting water loss
  • To correct the underlying disorder

Management

  • Mild asymptomatic hyponatraemia requires no treatment
  • Mild hyponatraemia with ECF volume contraction: Sodium releption with isotonic saline infusion
  • Hyponatraemia associated oedematous states:
    • Restriction of both sodium and water intake
    • Promotion of water loss in excess of sodium by use of a loop diuretic
  • For severe cases which are symptomatic (plasma sodium concentration <115 mmoles/L):
    • Hypertonic saline to raise sodium concentration by 1-2 mmol/L/hour for the first 3 hours, but not more than 12 mmoles/L during the first 24 hours

Calculation of the total amount of sodium to administer
Amount of sodium= (desired concentration – actual concentration) × body weight X 0.6